FIG. 17 shows an equivalent circuit of a pixel circuit of a typical active matrix liquid crystal display device. In addition, FIG. 18 shows a circuit arrangement example of an active matrix liquid crystal display device having m×n pixels. As shown in FIG. 18, a switch element composed of a thin film transistor (TFT) is provided in each of intersecting points of m source lines (data signal lines) and n scanning lines (scanning signal lines), and as shown in FIG. 17, a liquid crystal element LC and a retention capacitor Cs are connected in parallel through the TFT. The liquid crystal element LC is constituted by a laminated structure having a liquid crystal layer between a pixel electrode and an opposed electrode (common electrode). In addition, FIG. 18 shows the simplified pixel circuit only with the TFT and the pixel electrode (black rectangular section). The retention capacitor Cs has one end connected to the pixel electrode, and the other end connected to a capacity line CSL and stabilizes a voltage of pixel data held in the pixel electrode. The retention capacitor Cs prevents a voltage of the pixel data held in the pixel electrode from fluctuating due to a leak current of the TFT, a fluctuation of electric capacity of the liquid crystal element LC between a black display and a white display due to dielectric constant anisotropy of a liquid crystal molecule, and a voltage fluctuation caused through parasitic capacity between the pixel electrode and a peripheral wiring. By sequentially controlling the voltages of the scanning lines, the TFT connected to the one scanning line is set to a conducting state, and the voltage of the pixel data supplied to each source line is written in the corresponding pixel electrode by the scanning line.
In a normal display mode provided in a full-color display, even when display contents are composed of still images, the same display contents are repeatedly written in the same pixel while a polarity of a voltage applied to the liquid crystal element LC is reversed every one frame, so that the voltage of the pixel data held in the pixel electrode is updated, and a voltage fluctuation of the pixel data can be minimized. As a result, high-quality display of the still image can be realized.
Power consumption to drive the liquid crystal display device is dominated by power consumption to drive a source line by a source driver, and can be roughly expressed by a relational expression shown in the following formula 1. In the formula 1, P represents a power consumption, f represents a refresh rate (the number of refresh operations for one frame per unit time), C represent a load capacity driven by the source driver, V represents a drive voltage of the source driver, n represents the number of scanning lines, and m represents the number of source lines. In addition, the refresh operation means an operation to reset a fluctuation generated in the voltage (absolute value), which corresponds to the pixel data and is applied to the liquid crystal element LC, by writing the pixel data again, and restore it to an initial voltage state corresponding to the pixel data.P∝f·C·V2·n·m  (Formula 1)
Recently, along with the widespread of digital contents (such as an advertisement, news, and electronic books) associated with the progress of telecommunications infrastructure, the still image is required to be constantly displayed (a constant display mode), in an image display of the digital contents in a mobile terminal device such as a cell phone, or a mobile internet device (MID). The mobile terminal device which displays the digital contents uses a liquid crystal display device which is low in power consumption, but a time spent displaying the still image accounts for most of the operating time of the device, so that it is further required to reduce the power consumption while the still image is constantly displayed.
When the still image is constantly displayed, it is not always necessary to update the voltage of the pixel data every one frame because the display contents are composed of the still image. Therefore, in order to further reduce the power consumption of the liquid crystal display device, there has been an attempt to reduce a refresh frequency at the time of constantly displaying the still image. However, when the refresh frequency is reduced, the pixel data voltage held in the pixel electrode fluctuates due to the leak current of the TFT, or the like. Furthermore, an average potential during each frame period is also reduced, so that the voltage fluctuation leads to a fluctuation in display brightness of the pixel (transmittance of liquid crystal), and this is observed as a flicker. In addition, deterioration in display quality such that a sufficient contrast cannot be obtained could be caused.
Here, as a method of solving the problem that the display quality deteriorates due to the reduction in refresh frequency when the still image is constantly displayed, a configuration is disclosed in the following patent document 1. According to the configuration disclosed in the patent document 1, as shown in FIG. 19, a voltage reduction compensation circuit VLC is provided in the pixel circuit shown in FIG. 17, so that the reduction in pixel data voltage held in the pixel electrode can be collectively and automatically compensated with respect to each gradation for each pixel in the pixel circuit, without depending on an external refresh operation in which the source line and the scanning line are driven to externally write the pixel data voltage again. Thus, the refresh frequency can be reduced at the time of the constant display mode while the display quality is maintained, and in addition, the source line and the scanning line are kept in a halted state without being driven, so that the power consumption of a liquid crystal display panel can be considerably reduced.